While the glaciers were at their maximum extent, the ice nearly filled the canyons to their rims. Above, the uplands were probably blown nearly clear of snow in winter, though deep drifts accumulated on lee slopes. These drifts lasted well into the summer, in places possibly throughout the year, though they never became thick enough to form glaciers. Permafrost, or permanently frozen ground, probably formed where the snow cover failed to insulate the uplands. As the climate began to warm and the ice began its retreat, meltwater from the snow and permafrost penetrated fractures in the rock. Here, successive freezing and thawing broke up the rock and mixed with the overlying soil to form the mantle of angular blocks and sandy debris that is so characteristic of the uplands today. Frost heave tilted many slabs upward and, on gentle slopes, formed patterns in the ground. On steeper slopes, gravity caused the debris to slide downhill, giving the slopes a wavy expression or downhill streaming. Most of these deposits are now stable and overgrown with tundra, through which the blocks project.

Mantle of angular blocks and sandy debris on the gently sloping summit upland of Bighorn Flats, at 11,600 feet near Sprague Pass. (Fig. 31) (Gerald M. Richmond)

Exposure of the blocky upland mantle along Trail Ridge Road. (Fig. 32) (Gerald M. Richmond)

Patterned ground formed by frost-sorting of angular stones in blocky mantle on the north side of Trail Ridge Road east of Rock Cut. (Fig. 33) (Wayne B Alcorn)

In places, thawing ground and snow meltwater so fully saturated the stony slope deposits that they flowed slowly downhill. Examples of such solifluction (soil flowage) are widespread on Trail Ridge. Large bouldery solifluction terraces bench the slope of Sundance Mountain east of Forest Canyon Overlook. Smaller, less bouldery terraces form low treads on the slopes along Trail Ridge Road south of Iceberg Lake. Solifluction lobes occur below the road east of Rock Cut.

Soil flowage is no longer active over most of the uplands. However, here and there in the wetter places, frost heave, thaw, and soil flowage still go on today on a small scale. Some features of this activity are both curious and interesting. Many are to be seen near Trail Ridge Road. An active solifluction terrace lies on the slope below the road just east of Rock Cut. A group of irregular circles or polygons of stone slabs with centers of sandy material occur along the north side of the large curve east of Rock Cut, in the wet ground at the foot of a melting snowbank. The sorting process by which the slabs are separated from the sand actively goes on each summer.

Along the trail over the upland west of Iceberg Lake is a network of stone-filled channels a few feet wide in which the stones have been concentrated by frost heave and thaw from underlying and adjacent material containing both sand and stones. Narrow stone-filled channels, called block streams, can be seen from Iceberg Lake on the slopes of the upland to the north.

Bouldery solifluction terraces on slope of Sundance Mountain east of Forest Canyon Overlook. (Fig. 34) (George M. Richmond)

Active solifluction terrace below Trail Ridge Road east of Rock Cut. Note springs, ponds, and hummocks at back of terrace; frost boils on outer edge. (Fig. 35) (Wayne B. Alcorn)

Low rounded hills in the basin south of Fall River Pass are patterned by a series of closely spaced narrow treads that angle diagonally across their slopes like parallel trails. These are not made by animals, but rather by the slow flow of material directed laterally down the slope by tundra vegetation growing on the steep fronts of the treads—hence their name, turf-banked terraces. On the flatter tops of the hills active frost heave has formed a closely spaced pattern of circles, or "frost boils," in which the vegetation cannot get a foothold. Where the slopes steepen, these circular areas of frost activity flow together to form turf-banked terraces.

In valleys below the limits of the glaciers, the colder climate also induced frost heave and downslope movement of slope debris. Where there were cliffs of hard rock, freezing and thawing of moisture in the cracks pried loose blocks. These fell and accumulated at the base of the cliffs as cones or sheets of angular rock debris, called talus. Where the rock had been deeply weathered in preglacial time, as around Estes Park, soil flow at times of thaw stripped away the soft weathered sandy debris, leaving behind round, hard rock cores as giant boulders. These either remained behind in precariously balanced positions on the hills or tumbled down the slopes. They are abundant on the hills at the northeast edge of Estes Park and along the road north of town.

Active sorted polygons east of Rock Cut. Rock slabs in water are sorted from central sandy areas by frost heave and thaw. (Fig. 36) (Gerald M. Richmond)

Network of stone-filled channels separated by frost action from central grassy areas of stones mixed with sand. On widespread upland above Iceberg Lake. (Fig. 37) (Gerald M. Richmond)

Channel filled with blocks separated by frost action from intervening grass-covered areas of blocks mixed with sand. On slope north of Iceberg Lake. (Fig. 38) (Gerald M. Richmond)

Turf-banked terraces in basin south of Fall River Pass. (Fig. 39) (Gerald M. Richmond)

Under the cold conditions that existed during the advance of the glaciers, the forests retreated downslope and a wide spread tundra extended to altitudes perhaps as low as 8,000 feet. East of the mountains the forest spread far out on the High Plains where it became the habitat of mammoths, wooly rhinoceros, bison, and other large mammals.